1. Field of the Invention
This invention relates generally to a turbine assembly and particularly relates to a gas turbine engine assembly which incorporates thin structural support struts aligned with the exit swirl from a low pressure gas turbine, and which includes an array of outlet guide vanes mounted aft of the struts for deswirling the exhaust gasses.
2. Description of Prior Developments
The advent of high temperature turbine engines has increased the emphasis on single-stage high energy extraction turbine engine designs. Single stage turbine engines are particularly desirable in aircraft applications because single stage turbine engines are lower in weight and easier to maintain than counterpart multistage designs. However, single stage designs have traditionally posed several problems of their own including an increased amount of swirl or vorticity in the exhaust gasses. This swirl is particularly prevalent in medium and high bypass ratio designs where the ratio of the fan duct mass flow to the main duct mass flow is relatively high.
For increased bypass ratio turbine engines the low pressure turbine is subjected to increased loading resulting in high exit swirl. That is, the airflow exiting the low pressure turbine has a relatively high tangential velocity, in the order of thirty degrees or greater. In order to avoid large thrust losses, this swirl must be straightened before the hot exhaust gasses are discharged from the exhaust system.
One conventional approach to removing or straightening this swirl has been to mount a row of deswirl or outlet guide vanes directly behind the low pressure turbine rotor and ahead of or co-planar with the structural support struts and fairings. Another known approach combines the structural support struts and outlet guide vanes into a limited number of relatively long airfoil shaped members.
In addition to removing swirl from the gasses exiting the low pressure turbine it is also desirable to closely couple the high pressure and low pressure turbine bearing supports and associated structural frames to minimize the size and weight of the engine and to increase the stiffness of the low pressure rotor. By closely supporting or coupling the high pressure and low pressure turbine bearings, the axial distance between these bearings can be reduced and size and weight reductions can be achieved. It is further desirable to minimize the axial distance between the low pressure turbine rotor and the aft structural frame which supports the aft bearing for the same reasons, i.e., to achieve a reduced engine envelope and reduced weight and to increase rotor stiffness.
Prior single stage turbines have employed an array of outlet guide vanes located upstream or ahead of the aft bearing structural frame or have combined or integrated the outlet guide vanes within the aft bearing structural frame. Each approach presents its own drawbacks. By mounting the outlet guide vanes ahead of the aft bearing structural frame, the axial length of the rotor system must be increased since the aft bearing structural frame must be positioned further aft to accommodate the additional axial extent of the outlet guide vanes. Combining the outlet guide vanes with the aft bearing structural frame requires the presence of many long radially extending outlet guide vanes which are difficult to cool. This approach further requires wider or thicker outlet guide vanes for structural strength resulting in higher aerodynamic turning and drag pressure losses.
In order to increase the thrust output of a turbine engine an augmentor or afterburner may be provided downstream from the low pressure turbine, typically within the exhaust duct of the engine. Additional fuel may be injected into the exhaust duct and ignited to provide an additional high energy gas stream which may be mixed with fan bypass air and then ejected through an exhaust nozzle system to provide an extra high energy thrust output from the engine.
It is most important that the high velocity gases flowing into the afterburner have been previously straightened or deswirled by the outlet guide vanes so that these gases may be effectively diffused. The even and complete diffusion of the exhaust gases is necessary to maximize the combustion of the oxygen contained within the gases as they flow through the afterburner.
Some prior turbine engine designs located the outlet guide vanes across the exhaust flowpath ahead of mechanical support struts which formed part of the structural frame supporting the low pressure turbine aft bearing. These support struts not only supported the aft bearing and internal rotor shaft, but also provided internal passages for radially directing cooling airflow and engine oil through the support struts. Airfoil contoured shields covering the support struts reduced their aerodynamic drag and assisted the diffusion process. Although the outlet guide vanes removed the swirl, they nevertheless created large drag effects due to the high Mach number flow which is present directly adjacent the low pressure turbine exit where the outlet guide vanes were mounted.
Other prior turbine engine designs have used cambered airfoils to simultaneously deswirl the exhaust gasses and shield the mechanical support struts. Because of the large cross sections required of the mechanical support struts, these shielded designs required relatively thick shields with long axial lengths. These designs resulted in substantial drag effects and incomplete swirl removal. In effect, the thicker the strut supports and/or the outlet guide vanes, the greater the flow blockage, the greater the pressure loss and the greater the loss of thrust.